LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

L C L S C O N C E P T U A L D E S I G N R E P O R T
photons slowed down in the crystal and the electric field of the bunch propagating at a velocity
equal to c. A thin crystal with a low refractive index is therefore appropriate. The extremely high
electric field strengths of several GV/m from the 15-GeV, 1-nC LCLS bunch will afford use of
EO crystals with 100-µm thickness and relatively low electro-optic coefficients. For comparison
quartz has an EO coefficient as low as 0.2×10 −12 V/m with a refractive index of n = 1.5 compared
to a more usual EO material such as LiNbO3 with a coefficient of 30×10 −12 V/m and n = 2.3. It is
envisaged that such an electro-optic measurement station will be installed in the DL2 beamline to
measure the electron bunch length characteristics before it enters the undulator.
7.8.3 Beam Energy Spread Diagnostics
7.8.3.1 DL1 Energy Spread Diagnostics
The energy spread measurement in DL1 is made with a single profile monitor (OTR monitor)
located between the two dipoles of DL1 at the point where the horizontal dispersion function
reaches a value of ηx ≈ –153 mm with a horizontal beta function of βx ≈ 0.79 m (at S ≈ 18.5 m in
Figure 7.33). For the nominal emittance and nominal energy spread of σδ ≈ 0.10% at 150 MeV,
the betatron beam size is 52 µm, but the dispersive size is 153 µm. This produces a systematic
energy spread measurement error of 6%. The statistical error depends on the profile monitor and
should be 5–10%.
7.8.3.2 BC1 Energy Spread Diagnostics
The energy-spread measurement in BC1 is made with a profile monitor (phosphor) located in
the center of the chicane at a point where the horizontal dispersion function is ηx ≈ 228 mm and
the horizontal beta function converges towards a minimum of βx ≈ 6.5 m (at S ≈ 34.8 m in Figure
7.19). For the nominal emittance and nominal energy spread of σδ ≈ 1.78% at 250 MeV, the
betatron beam size is 115 µm, but the dispersive size is 4.1 mm. This produces no systematic
error in the energy-spread measurement. The collimator jaws in BC1, upstream of the profile
monitor, can be used to select energy bands for diagnostic purposes.
7.8.3.3 BC2 Energy Spread Diagnostics
The energy-spread measurement in BC2 is also made with a single profile monitor
(phosphor) located in the center of the first chicane at a point where the horizontal dispersion
function is ηx ≈ 341 mm and the beta function converges towards a minimum of βx ≈ 25 m (at
S ≈ 411 m in Figure 7.26). For the nominal emittance and nominal energy spread of σδ ≈ 0.76%
at 4.54 GeV, the betatron beam size is 53 µm, but the dispersive size is 2.6 mm. As in the case of
BC1, this produces no systematic error in the energy-spread measurement. As in BC1, the
collimator jaws in BC2, upstream of the profile monitor, can also be used for diagnostic purposes.
7.8.3.4 DL2 Energy Spread Diagnostics
The energy spread measurement in DL2 is made with a profile monitor (a retractable OTR
monitor) located where the dispersion function is ηx ≈ 91 mm and the beta function converges
towards a minimum of βx ≈ 4.0 m (at S ≈ 1236 m in Figure 7.36). For the nominal emittance and
a core rms energy spread of σδ ≈ 0.03%, the betatron beam size is 12 µm, but the dispersive size
7-100 ♦ A C C E L E R A T O R